Multifilamentary MgB2-based solenoidal and racetrack coils

In a continuation of a series of reports on the fabrication and properties of several demonstration coils and solenoids wound with MgB2 strand this paper describes the winding, testing, and load-line comparisons for a racetrack coil (R-4) and two solenoids (S-1 and S-2). Racetrack R-4 was wound with 19-filament strand, S-1 and S-2 with 7-filament strand. The filaments were clad in Nb-lined Cu and the restack bundle was enclosed in a monel sheath; some Cu stabilization was included. Transport Jc measurements were performed initially in liquid He, and then as a function of temperature up to 30 K. Racetrack R-4 achieved 240 A in self-field at 7.7 K corresponding to a Jc of 240 kA/cm2, leading to a strand Je of 43 kA/cm2, and a winding Je,w of 22 kA/cm2. At 4 K, S-1 (310 m of strand) achieved an Ic of 138 A and a bore field of 3.4 T; S-2 (742 m of strand) an Ic of 85 A and a bore field of 3.9 T. The magnetic field profiles for all coils were calculated using a finite element approach in conjunction with the Biot–Savart equation. The windings were treated as a series of small segments, each tangential to the winding, numerical errors were estimated. These calculations were used to generate load-line comparisons for the coils.